Electric pulse generator



Jan. 8,1952 A. H. REEVES ELECTRIC PULSE GENERATOR Filed March 18, 1948 Attorney Patented Jan. 8, 1952 UNITED STATE S PATENT, OFFICE.

ELECTRIC PULSE GENERATOR Application March 18, 1948, Serial No. 15,581 In Great Britain March 20, 1947 Claims.

The present invention relates to an electric pulse distributor or modulator circuit arrangement employing a cold cathode gas filled discharge tube.

The arrangement employs a discharge tube of the kind described in the specification of my copending U. S. application No. 14,184/48 filed March 11, 1948 for Improvements in or relating to Electric Discharge Tubes.

An electric pulse distributing arrangement according to the invention comprises a cold cathode gas discharge tube having a sequence of discharge gaps, means for applying a potential to each of the gaps of sufllcient magnitude to maintain a discharge thereacross, but insufiicient by itself to initiate a discharge, means for applying a pulse to fire the first gap of the sequence, the arrangements being such that the remaining gaps are conditioned by ionisation migration to fire spontaneously in turn at one at a time, and means for deriving output pulses from the gaps.

In order to illustrate the invention, a specific application of the arrangement will be described, but the invention is not limited to this specific application, and may be adapted for use in other circumstances.

The-invention will be described with reference to the drawing accompanying the provisional specification in which:

Fig. 1 shows a diagram of a pulse train which may be produced by the circuit arrangement according to the invention:-

Fig. 2 shows a schematic circuit diagram of an arrangement according to the invention; and

Figs. 3 and 4 show minor modifications of Fig. 2

to generate a pulse period modulated train of pulses for an electric communication system of the type described in the specification of U. S. application No. 756,262 filed June 21, 1947, for Improvements in or relating to Multiplex Electric Communication Svstems by P. K. ChatterjeaA. H. Reeves. This type of train is illustrated in Fig. 1. From this figure it can be seen that the signalling time is divided into a number of equal signalling periods by a train of regularly repeated pulses 0 having some characteristic by which they may be distinguished from the channel pulses, such, for example, as a larger amplitude, as indicated in Fig. 1. will be called "coded pulses for convenience, and they do not carry any signal. Three successive coded pulses II are shown in Fig. 1. The chan- The circuit arrangement of Fig. 2 is adapted Such pulses nel pulses are initially generated as trains of pulses having a repetition frequency the same as that of the coded pulses and are interleaved so that in each of the above mentioned signalling periods each pulse belongs to a different channel. l are shown in each of the two signalling periods which are shown in Fig. 1. The time positions of the channel pulses are varied by the corresponding modulating signals so that the time spacing between the pulse I of channel I and the preceding coded pulse 0 depends only on the instantaneous voltage of the signal carried by channel I, and so that in the case of any channel pulse 2, the time spacing between that pulse 2 and the preceding pulse I depends only'on the instantaneous voltage of the signal carried by channel 2, and so on. It will be understood, of course, that there may be any number of channel pulses in each signalling period.

In Fig. 1 the unmodulated positions of the channel pulses are shown by the dotted lines, and these positions should preferably be equally spaced between two successive coded pulses. The channel pulses I, 2, 3. 4 are shown variously displaced in the signalling periods in accordance with the corresponding si nal volta es.

The arran ement of Fig. 2 emplovs a multigap cold cathode gas filled dischar e tube which as alreadv stated is of the tvpe described in the specification oi' co-pending U. S. Application No. 14.184/48. It comprises an anode I I! which consists of a long straight metal p ate. and arran ed opposite the anode are a p uralitv of similar metal cathodes consisting of short cvlinders, and forming t erewith a seouence of discharge gaps. These cathodes are equally spaced apart in a straight line and the first of them III is the starting cathode and the remainder are channel cathodes and are used for generating the channel pulses I, 2, 3 etc. Only the first two and the last of these are shown and are designated IIIA, H43 and IIlN. It will be understood that there is one of these channel cathodes for each channel oi the system. The channel cathodes are all enually spaced from the anode H3. but the starting cathode III is preferably placed rather closer to the anode.

The starting cathode is connected to ground through an adjustable resistance IIS. and the channel cathodes are connected to ground through the secondary windings oi corresponding transformers IIBA, IIBB etc. and HON. Resistances II2A, II 23 etc. and |I2N are connected in series with these secondary windings as shown.

Four channel pulses designated I, 2, I and I The anode H3 is connected through the primary winding of an output transformer II8 to the positive terminal II! for the high tension source, the corresponding negative terminal I being grounded. The secondary winding of this transformer'is connected through a network consisting of a series rectifier I2I and two shunt resistances I22 and I23 to a pair of output termi- Each time a gap is fired, the anode current which passes through the primary winding of the transformer I I8 increases abruptly by a constant amount, and since thetransformer acts as a differentiating device, a train of short pulses of one sign will be produced in the secondary winding of the transformer, which should be poled so that these output pulses will pass through the nals I24 which are connected to the line or other medium over which the pulses are to be transmitted.

A master pulse generator I25 generates two trains of short negative pulses having the same repetition frequency, of 10,000 pulses per second, for example. The first of these trains comprises negative pulses of large amplitude (which will be called extinguishing pulses),' and is applied through the blocking condenser I26 to the anode I I3. The second train'comprises negative pulses,

'of moderate amplitude (called starting pulses) each of which occurs very shortly after the corresponding pulse of the first train and is applied over the blocking condenser I21 to the starting cathode Ill. The master pulse generator I25 may be of any suitable known type and does not need detailed description. v

The potential of the high tension operating source connected to terminals I I9 and I20 should .be insufficient to start a discharge between any of the cathodes and the anode, but should be capable of maintaining a discharge when it has been started. Then the circuit operates as folflows:

It will be assumed that a negative extinguishing pulse has just been applied to the anode H3.

and is of sufficient amplitude to extinguish any discharges which may exist between any of the cathodes and the anode. pulse which immediately follows should be of sufficient amplitude to initiate a discharge between the cathode [I4 and the anode 3. It is explained in the specification of the co-pending U. S. Application No. l4184/48,'that under the proper conditions of gas pr-essure,.'electrode spacing and other factors, the presence of the discharge from the cathode II 4 progressively lowers by ionisationmigration the minimum striking potential of any other gap by an amount, and at a rate which depends amongother factors on the distance between the two gaps. Thus for example, the striking voltage of the gap corresponding to the cathode 'I MA may be lowered by 80 volts more than that corresponding to the next cathode IIlB is lowered. It follows that a short time after the discharge .from the cathode Ill has struck, a discharge will occur from the oathode I HA, but not from any of the other cathodes, becausethe striking voltage will not have been lowered sufllciently since they are further away. However, the discharge from cathode I HA acts 'in exactly the same way on the striking voltage correspondingto the next cathode 4B; and a discharge takes place from that cathode; 'It will be clear that the same process continues, each gap being fired shortly after the preceding gap until the last cathode I I 4N is reached.

.Hence since the various cathodes are equally spaced, the gaps will be fired in succession at substantially equally. spaced intervals of time, as soon as the discharge from the cathode .I I4 has been struck by the starting pulse.

The negative starting sign on the signal voltage.

extinguishingpulse which extinguishes all the gaps, and the tube is then ready for a fresh series of operations which commences with the starting pulse which immediately follows. Since this extinguishing pulse cuts off the current through the primary winding of the transformer, a short pulse of large amplitude but of opposite sign to the output pulses will be generated, and this is cut off by the rectifier I2I. However, it might be useful to transmit this large pulse for synchronising purposes, for example, and in that case the rectifier I2I and resistances I22 and I23 could be omitted.

It will be apparent that the above mentioned output pulses will comprise the coded pulses 0 of Fig. 1, derived from the starting cathode Ill, and the channel pulses I, 2, 3, etc., derived in turn from the cathodes INA, 413, etc. and IIlN. The minimum striking voltage of any particular cathode such a I B falls substantially uniformly with time when the preceding gap has been fired. Thecorresponding gap will therefore be fired when the striking voltage has fallen to the potential of the high tension source. If,

- however, a modulating signal is applied to terminals 13, a voltage will be produced in the secondary winding of the transformer I IBB which increases or diminishes the effective operatingvoltage applied to the corresponding gap. The discharge therefore occurs earlier or later than usual by an amount depending in magnitude and In other words, the time interval between the striking of the discharges from the cathodes I MB and I HA is modulated in accordance with the signal, and so the time interval between the corresponding channel pulses 2 and I (Fig. 1 is likewise modulated. It will be clear that all theother time intervals will be modulated in a similar way by the signals applied to the corresponding terminals I I1.

It should be explained that the delay in the striking of the discharges from the cathodes is believed to be principally due to the following circumstances:

(a) A small but finite time is necessary for the cathode glow which starts at the tip of the cathode to'extend along the whole length of the cathode. Thus the lowering of the striking voltage of the adjacent gap which depend on the increasing numbers of ions and electrons produced takes time to develop.

(b) The ions and electron formed in the first gap take a small but finite time to drift into the neighbourhood of the second gap, and so their effect on the second gap is delayed.

This ionisation coupling which results from these two effects therefore has not, so to speak, an infinitely steep leading edge, but increases progressively from zero, and so the resulting lowering of the striking voltage of the second-gap increases with time in a finite and continuous manner.

For this reason it is possible-to modulate the timed so that striking time of the second gap within certain limits by an applied signal voltage in the manner explained.

The initial conditions for the striking of the discharge from the first cathode I H may be conveniently adjusted by varying the resistance H5, which varies the rate of increase and final value of the corresponding current.

The gas mixture, pressure, and cathode gaps and spacings must of course be designed in accordance with the pulse spacings and time modulations desired. An example is given below of design details of a tube suitable for generating a pulse train according to the present invention. The repetition frequency of the coded pulses is assumed to be 10,000 pulses per second, and the time interval between any two adjacent pulses (Fig. 1) is assumed to be between 3 and 5 microseconds. The details of the tube and its operation are as follows- Gas filling:

Neon 92% .Argon 1% Total pressure 100 mm. Hg.

Hydrogen '7 Electrode Material: Nickel cathode length millimetres 2 Spacing between each pair of adjacent cathodes millimetres 2 Gap between cathode H4 and anode do 2 Gap between each of the other cathodes and anode millimetres 2 Voltage of operating source 7 volts 180 Peak value of each cathode current millamps 2 Duration of extinguishing pulse microseconds 2O Amplitude of extinguishing pulse volts 150 Duration of starting pulse microsecond 1 Amplitude of starting pulse volts 20 With this tube, the extinguishing time for all the gaps will be about 10 microseconds, so the duration of the extinguishing pulse is chosen to be about double to give a good margin of safety.

While the tube shown in Fig. 2 has a flat straight anode plate and the cathodes are arranged in a straight line the anode could be curved, and take other forms than that of a plate, and the cathodes could be arranged in other ways.

An improved control may also be obtained by the use of an extra cathode providing an additional priming gap across which a discharge is permanently maintained. Such a priming gap should preferably be equidistant from the other cathodes, which therefore should be arranged in a circle with the extra cathode at the centre as shown diagrammatically, for example, in Fig. 3, which gives a plan view of a modification of Fig. 2. Elements not shown in Fig. 3 may be arranged exactly as in Fig. 2. The use of a permanently discharging priming gap is described in thespecification of co-pending application No. 12086/47, now abandoned.

In this case the anode H3 consists of a circular metal disc, and the necessary number of cathodes H4 are arranged concentrically therewith, and with the proper spacing therefrom. as already explained. The extra cathode I28 is shown in the centre, and is connected through an adjustable resistance I29 and an additional voltage source I30 to terminal I2ll. The cathodes H4 are supposed to be connected as shown in Fig- 2.

The extra cathode is so spaced from the anode H3 that a discharge takes place permanently therefrom. The additional source is connected to augment the operating voltage for this cathode, and should be of such voltage that the dis-: charge is not extinguished by the extinguishing pulse applied through condenser anode H3.

By adjusting the current through the cathode I28 by means of the resistance I29, the general ionisation level of all the other cathodes may be varied. An increase in the general ionisation level has four principal effects on the other gaps:

1. A reduction of the minimum striking volt,-

age;

2. A reduction of the difference between the striking and maintaining voltages;

3. A reduction in the minimum extinguishing time, and

4. An increase in the rate of spread of the cathode glow along each cathode when the discharge is struck.

Eifect No. 3 is very useful as it is desirable to reduce the extinguishing time in order to obtain the maximum channel space, and effect No. 4 is one factor which determines the relation between the signal voltage and the resulting time modulation, and can be used to adjust this relation.

Fig. 4 shows a slight. modification of the arrangement of Fig. 3. The extra cathode I28 is provided with a corresponding extra anode I3I which is connected separately to the positive hi h tension terminal H9. The additional source I 30 shown in Fig. 3 is then unnecessary, since the extinguishing pulses will not affect the .extra anode. In Fig. 4 the anode H3 has been shown in the form of an annular disc, and the extra anode I3I is a centrallyplaced circulardisc insulated from the anode H3. As in the case of Fig. 3, the ionisation level may be adjusted by means of the resistance I29.

. In Figs. 3 and 4, the last of the channel cathodes. IMN should be well separated from the starting cathode H4, as indicated, to prevent it from being appreciably affected by the ionisation from the starting gap, so that premature firing of the last gap is avoided.

By a slight modification, the arrangement shown in Fig. 2 may be adapted to operate as a pulse distributor for a multi-channel timephase modulation pulse communication system, for example. In this case the transformer H8 may be omitted, the anode I I3 being connected directly to terminal H9. After the application of the starting pulse to the cathode H4, the remaining gaps are fired in turn at regular intervals and each therefore generates a short pulse which may be obtained from the corresponding transformer HIA, I I'IB, etc. and I I'IN.

These transformers may therefore be connected to corresponding pulse time phase modulators (not shown) of conventional pattern instead of to the signal sources, and corresponding modulated trains of pulses may be obtained from these modulators properly timed for interleaving in the usual way. If desired the cathode H4 may be provided with a corresponding transformer (not shown) in order to obtain the coded pulses which define the signalling periods.

It will be clear, also that a distributor of this kind may be used to provide the gating pulses at the receiver of a multi channel time phase modulation system.

I26 to the assnm f" It'willalso be evident that the-pulses generated by the tube shown in Fig. '2 may be produced it desiredat various unequal intervals byspacing the cathodes ill in a corresponding unequal manner. v a What'is claimed-is:

1. An electric pulse' modulatorcomprising a discharge device having a discharge gap, means for gradually lowering the resistance or said gap, means for applying a voltage to said gap while said resistance is being lowered to a value suificient to cause said gap to fire when the resistance has reached a predetermined level, means for applying a signal voltage to said gap for varying the effective voltage on said gap and the time or firing thereof, and means responsive to the firing of said gap for deriving an output pulse.

' 2. A modulator according to claim 1, wherein said device is a gaseous discharge tube and said means for gradually lowering the resistance of said gap comprises means for gradually ionizing I the 'gas in said gap. 1 r V 3. A modulator according to claim 1, wherein said discharge device is a gaseous discharge tube and said means for gradually lowering the resistance of said gap comprising a priming discharge gap, means for firing said priming gap,

. said priming gap being arranged adjacent said first mentioned gap so that the ionization pro- .duced by the firing of the priming gap gradually migrates toward said first mentioned gap and gradually lowers its resistance.

'4. A modulator according to claim 3, further including means for extinguishing said gap discharges.

5. A multichannel pulse modulation system comprising a discharge device including a plurality of successive discharge gaps arranged so that the resistance of each gap is gradually lowered upon the firing of its preceding gap, means .for firing the first gap, means for applying a voltage to each of the other gaps while said resistance is being gradually lowered to a value sufllcient to ca the firing of each of said other gaps at a predetermined time after the firing of its preceding gap, means for applying ing to claim 5, wherein said discharge device is 8 a gaseous discharge tube and said gaps are arranged adjacent each other so that the ionization produced by the firing o! a given gap gradually migrates to the next succeeding gap and lowers the resistance thereof. I

7. A multichannel pulse modulation system according to claim 5 in whichiall said other gaps have one electrode in common and said output pulses are derived from the common electrode.

8. A multichannel pulse modulation system ac-,

cording to claim 5 in which all said other gaps have a common anode, further including means eliminating the extinguishing pulse from. the

output.

10. A multichannel pulse modulation system comprising a discharge device including a plurality of successive discharge gaps arranged so that the resistance of each gap is gradually, lowered upon the firing 01' its preceding gap, means for firing the first gap, means for applying a voltage to each 01' the other gaps while said resistance is being gradually lowered to a value sumcient' to cause the firing of each of said other gaps at a predetermined time after the firing of its preceding gap, means for applying signal voltages of different channels to each of said other gaps to vary the efiective voltage thereacross and their time of firing with respect to their preceding gaps, and means for deriving output pulses from the firing 01' said other gaps, all said other gaps having one electrode in common, said output pulses being derived from the common electrode, and said means for deriving output pulses from the firing of said other gaps comprising an output transformer having its primary arranged in series with said common electrode. ALEC HARLEY REEVES.

REFERENCES CITED The following references are oi. record in the file oi. this patent:

UNITED STATES PATENTS Number Name Date 1,863,278 Nicolson June 14, 1932 2,088,474 Haller July 27, 1937 2,404,920 Overbeck July 30, 1946 2,443,407 Wales June 15, 1948 

